citemplate: make it so docker builds only run for detached pipelines

Currently, if you have a MR open, there 2 pipelines being
triggered. One normal, and one detached.

Previously, if you were to rebuild an image, the jobs of the
docker build stage would be executed concurrently, race
and end up both rebuilding the image.

Make them manual for normal pipelines to avoid such occurrence
and waste of resources.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-ci/-/merge_requests/320>

gitlab/ci_template.yml

@@ -195,7 +195,14 @@ default:
 .base:
   image: "$CI_REGISTRY/freedesktop/ci-templates/buildah:2020-03-24"
   rules:
-    - when: 'always'
+    # Always run for detached pipeleines
+    - if: '$CI_MERGE_REQUEST_ID'
+      when: 'always'
+    - if: '$CI_COMMIT_BRANCH == "master"'
+      when: 'always'
+    - if: '$CI_COMMIT_BRANCH == "$GST_UPS_BRANCH"'
+      when: 'always'
+    - when: 'manual'
   variables:
     STORAGE_DRIVER: 'vfs'
     BUILDAH_FORMAT: 'docker'